In the past, vulcanized rubber or synthetic rubber elastic bands or threads have been used to provide elastic properties to flexible substrates by attaching the elastic to the substrate using materials such as thread, yarn or adhesive in a sewing, weaving or adhesive process. The attachment of elastic bands to the underlying flexible substrate consumes additional materials and manufacturing overhead and poses substantial problems in the industry. Natural vulcanized or crosslinked synthetic rubbers are difficult to feed continuously and at high speeds in view of their tendency to stretch and relax during mechanical processes, resulting in articles with broken elastics, articles having an elastic with too great or too little tension or articles with partly attached elastics. Further, adhesives which have been used in the past to bond elastic bands to a flexible substrate generally have had poor adhesion to the elastic bands, resulting in the separation of the elastic during any substantial flexing of the substrate.
Buell, U.S. Pat. No. 4,081,301, issued Mar. 28, 1978, developed a process for attaching elastic leg bands to substrates, in which adhesive is applied along the length of continuous bands of elastic which are in a stretched condition, to a continuous web of substrate. The patent further suggests that the elastic can be coated with a heat-activated non-pressure sensitive adhesive prior to contact with the web. Still another suggestion relates to the use of a heat-sealable non-pressure sensitive elastic ribbon which can be adhered to the web with the aid of a suitable heating means.
Accordingly, a need exists for a self-adhering elastic band which can be continuously extruded and applied to flexible substrates at high speeds using automatic machines. A further need exists for a self-adhering elastic which during flex will resist detachment from the substrate. Another need exists for a self-adhering elastic having adequate tensile strength which can be attached with strong bonds to a flexible substrate at high machine speed without breaking.
Traditional elastic materials are generally crosslinked, three-dimensional networks of vulcanized natural or synthetic rubber. The crosslinked three-dimensional structure comprises a reversible energy storing network. Stress applied to the substance results in a strain or deformation of the three-dimensional network which stores energy, applied during stress, which can be spontaneously substantially recovered upon the removal of the stress. Elastic substances are ideal for imparting gathers to flexible substrates since they lengthen and contract in a constant and predictable manner through a wide temperature range.
Pressure sensitive adhesives, on the other hand, require a different set of properties. Upon the application of stress or force to a pressure sensitive adhesive, in the form of pressure, the adhesive must deform in order to come into intimate contact through viscous flow with the surface of a substrate in order to form adhesive bonds by Vander Walls attraction. Upon removal of the stress or pressure in order to preserve the adhesive bond, the adhesive must not recover from the deformation. Substances that are pressure-sensitive adhesives exhibit viscous flow and therefore inherently do not substantially recover from such deformation.
It is therefore apparent that elastic materials have minimal adhesive properties and pressure sensitive adhesives have minimal elastic properties. Commonly available pressure sensitive adhesive or elastic materials do not have the correct balance of properties which would result in a self-adhering elastic material since the molecular properties that result in elasticity are those that commonly result in the absence of adhesive properties. Only a unique combination of molecular properties can combine substantial elasticity and adhesive properties in a single composition.
See Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Edition, John Wiley & Sons, Inc., New York, N.Y., 1963, Volume 1, pp. 371-404, and Volume 7, pp. 676-705 for a discussion of elastics and adhesives.